Purified native dextrans, crude native dextran and clinical dextrans did not sensitize guinea pigs actively. Dextran sensitivity could be induced in guinea pigs by inoculation with Leuconostoc mesenteroides NRRL strains B-1374, B-1377, B-1424 and Leuconostoc dextranicum, strain B-1375. However, no sensitivity could be induced with Leuconostoc mesenteroides B-512(F). The sensitivity induced in guinea pigs by Leuconostoc mesenteroides could be inhibited by prior administration of dextran. Antidextran rabbit serum was produced by immunizing rabbits with Leuconostoc dextranicum B-1375 and Leuconostoc mesenteroides B-1424. Rabbits immunized with Leuconostoc mesenteroides B-512(F), without adjuvants, produced antiserum to a minor component of dextran, probably a somatic antigen. Rabbits immunized with B-512(F) with adjuvants produced antibodies both to dextran and to the minor component. The precipitating capacity of dextran with antiserum diminished with deceased molecular size. The lowest fraction with a molecular weight of 10,700 precipitated about one-half of the antibody content of the serum. Hydrolysis increased the shocking capacity of native dextran in passively sensitized guinea pigs. The most effective molecular size was between 41,600 and 35,400.